1. Field of the Invention
The present invention relates to offset printing inks and, more particularly, to invisible, infrared offset printing inks and methods of making such inks which, in printed form, can be detected only with the help of suitable infrared light sources.
2. Description of the Prior Art
Infrared (IR) absorbing dyes have numerous applications, such as optical recording systems, thermal writing displays, laser printers, laser filters, infrared photography, medical applications and printing, for example, of bar codes. With regard to printing applications, some of the existing devices exploit the process of fluorescence in which a dye is excited by ultra-violet (UV), visible or near-IR radiation and fluorescence light emitted by the dye material is detected. For example, Andrus et al., in U.S. Pat. No. 5,093,147, disclose a method of providing intelligible markings. This reference teaches a jet printing process used to apply a compatible liquid or viscous substance containing an organic laser dye that is poorly absorptive of radiation in the visible wavelength range of about 400 nm to about 700 nm, and is highly absorptive of radiation in the near-IR wavelength range of about 750 nm to about 900 nm. The dye fluoresces at longer wavelengths in the IR in response to radiation excitation in the near-IR range.
Near IR absorbing dyes have been used in a variety of applications, including optical recording media, printers, displays, filters, infrared photography, photosensing and medical and non-linear optics applications. Typically, it is desirable for the dyes used in these applications to have strong absorption in near-IR at the emission wavelengths of semiconductor lasers, light fastness, and resistance to chemical and thermal damage. It has been reported by Ueno and Yuasa, Infrared Absorbing Dyes (1990), that naphthalocyanine derivatives address these requirements and can be used in the above applications. A variety of applications and methods employed to synthesize naphthalocyanines are described by Tai et al., in EP Application No. 0344891, and Itoh et al., in EP patent Application No. 0313943.
Printing inks, including those used to provide intelligible markings, are made from dispersions of pigments, or solutions of dyes, in a carrier vehicle which forms a fluid, paste, or powder to be applied to and dried on a substrate. Inks are typically comprised of four material categories, including: (a) colorants, which include pigments, toners and dyes, to provide the color contrast with the substrate; (b) vehicles, or varnishes, which act as carriers for the colorants during the printing operation, and bind the colorants to the substrate upon drying; (c) solvents, which primarily assist in the formation of the vehicle, and reduce ink viscosity; and (d) additives, which influence the printability, film characteristics, drying speed, and end-use properties. It has been found that, generally, the most important properties of all inks are drying, printability (a function of the rheology of the ink), and color. Printing inks are typically applied in thin films on a wide variety of substrates, such as paper, paperboard, metal sheets and metallic foil, plastic films, and molded plastic articles, textiles, and glass. Generally, there are four broad classes of printing inks, offset lithographic (planographic), letterpress, flexographic, and rotogravure, based on the mechanism of the particular printing process and the rheology of the ink. These inks vary in physical appearance, composition, method of application, and drying mechanism.
There are many types of offset lithographic (planographic) inks, depending upon the press equipment, substrate and applications. Offset lithographic printing typically utilizes a photochemically treated plate, whose image areas accept ink and whose nonimage areas reject ink but accept acidified water (typically referred to as a fountain solution). The printed image is first transferred to a rubber drum (blanket) from the treated plate and is then applied to the substrate. This type of printing is done with water-repellant and acid-resistant paste inks.
An offset lithographic ink should not transfer pigment (bleed) to the acidic fountain solution, depending upon the extent to which the vehicle wets the pigment. Also, although the inks typically have a much higher viscosity than the fountain solution, depending upon the interfacial relationship between the ink and fountain solution, the ink and fountain solution should not be easily emulsified. Additionally, the rheology of the ink should be compatible with the particular type of offset printing machine on which printing is done (for example, sheet-fed or web-fed), particularly with respect to the speed of the process. Other requirements of lithographic inks involve drying properties, and resistance to heat and light.
An offset lithographic ink to be read at an infrared wavelength (between about 780 nm and about 1,800 nm), in the presence of a background of visible light, preferably should have a high extinction ratio at the peak of the absorption wavelength with very little absorption in the visible light wavelengths. The ink should also have physical, mechanical and chemical compatibility with the printing ink base and printing mechanism. It would also be desirable to produce an offset lithographic infrared printing ink which is non-toxic to users and formulators. While many ink formulations exist which strongly absorb IR wavelengths, to date, none address all of the foregoing requirements.
Accordingly, it is an object of the present invention to provide an offset lithographic infrared printing ink having an extinction ratio of at least 20:1 at the peak of the absorption wavelength, and very little absorption in the visible light wavelengths.
It is a further object of the present invention to provide an offset lithographic infrared printing ink having physical, mechanical, and chemical compatibility with the printing ink base and printing mechanism.